I use a palm and notepad for all my tasks, so I'm limited by the 16 category limit, which is fine for me. This is still a work in progress, but here's my contexts at the moment:

@agendas
@calls
@datacenter #physical stuff i want to do the next time I'm at the datacenter (about 20 miles from our office/my house)
@errands
@home
@internet
@read/review
@someday/maybe
@waiting
@work

Non GTD categories:
Business
Personal # these could both be expanded to Business Ref and Personal Ref
PalmWiki # all the items I create for the nice PalmWiki app

@agendas is probably used a little differently than it's proper meaning. I use it more for things that I've lent out to people. For example a friend had car trouble and I lent him some money, I have a 'money bob owes...' and mention everything in there. So it's more of a list of stuff I should remind someone else the next time I see them.

The other interesting thing I've done on the palm is the use of "..." for note titles to indicate there's more to it than a single line. For example I have an item "set up auto-pay for cc". I have another item "set up auto-pay for dentist...". I know that there's more information for the dentist auto-pay (how much I still owe) where the credit card is just going to be a set recurring amount of whatever and it will be a few years before it's paid off, so no need for any extra information, which is why it doesn't have the "..." at the end. Also the pda will automatically put ... at the end if I make the title to long. So whether it's done automatically or by me, it get the same idea across that "hey, there's more to this task than what you're seeing"